The present invention relates to a process for using nitric acid to oxidize polyarylene sulfide to polyarylene sulfoxide, oxidizing at least 98% of the sulfide groups of the polyarylene sulfide to sulfoxide groups.
Polyarylene sulfoxides are specialty engineering polymers, applied in particular where high requirements are placed on the polymeric material. For example, polyarylene sulfoxides have high glass transition temperatures, high melting points and high decomposition temperatures. Polyarylene sulfoxides are moreover of particular technical interest to the extent that they can be used as high-temperature blowing agents for producing plastic foams.
Despite these many advantageous material properties, polyarylene sulfoxides have not so far been very widely used since these polymers are industrially complicated to prepare and the material is therefore relatively expensive.
In the prior art pure polyarylene sulfoxide, having a proportion of xe2x89xa799% of sulfoxide groups in the polymer chain, is synthesized by introducing polyarylene sulfide powder into 86% strength nitric acid. During this reaction the polyarylene sulfide dissolves with vigorous evolution of nitrous gases, ensuring complete oxidation, and then has to be isolated by precipitation in water. This synthetic route was described by Dr. M. Peter in his dissertation xe2x80x9cSynthese und Strukturmodifizierung von Poly(thio-1,4-phenylen)xe2x80x9d [Synthesis and structural modification of poly(thio-1,4 phenylene)], prepared between 1992 and 1994 in Prof. Heitz""s group at the University of Marburg.
The known process, which initially served merely to characterize the polyarylene sulfide used, was subsequently optimized for the preparation of sample quantities of polyarylene sulfoxide to the extent that it was discovered that from about 130 to 150 units by weight of PPS powder (PPS=polyphenylene sulfide) can be dissolved in 600 units by volume of 86% strength HNO3. The viscous solution can then be introduced dropwise with vigorous stirring (using an Ultraturrax) into a large excess of water, the precipitated PPSO (PPSO=polyphenylene sulfoxide) reaction product filtered off with suction, washed until neutral and dried.
However, this process known as the HNO3 process is not very suitable for conversion to industrial scale for production purposes, since the complicated isolation of the product from the reaction mixture makes the process expensive and therefore not cost-effective. The industrial handling of nitric acid at this high concentration is also very complicated. Other versions of processes for preparing PPSO by oxidizing PPS were therefore proposed.
One other way is to oxidize PPS using N2O4 in an autoclave, directly in a gas/solid phase reaction, or with methylene chloride as suspension medium, or in dichloroacetic acid as solvent. However, the latter procedure is hardly different from the known HNO3 process, since here, too, the PPSO dissolves and is troublesome to isolate (EP-A-0 791 027).
Another way is given by a process in which the oxidation is carried out using H2O2 in sulfuric acid (xe2x86x92Caro""s acid) or dichloroacetic acid (xe2x86x92dichloroperacetic acid) under specific conditions. This is described in DE-A 197 51 239.9.
Of the processes mentioned above the process in sulfuric acid using H2O2 as oxidant has shown itself to be of particular industrial interest due to the quality of the resultant product in terms of degree of oxidation and weight loss on decomposition, and to its feasibility of industrial application, specifically with regard to the handling of the substances used and released.
However, a disadvantage is that with the H2SO4/H2O2 process, unlike in the HNO3 process, very careful attention has to be paid to the stoichiometry of the reaction components and moreover to the feeding of the oxidant in very small steps, otherwise there is a risk of excessive oxidation and the formation of undesirable sulfone groups.
It was therefore an object of the present invention to provide a process which prepares polyarylene sulfoxide by oxidizing polyarylene sulfide, gives a good yield of pure polyarylene sulfoxide and which is easily handled industrially to the extent that the resultant product, polyarylene sulfoxide, can be isolated from the reaction mixture by simple means.
This object has been achieved by a process of the type mentioned at the outset, the characterizing feature of which is that polyarylene sulfide is treated with nitric acid at a concentration of from 60 to 80%, with heating.
The polyarylene sulfide preferably used according to the invention is polyphenylene sulfide (PPS). However, according to the invention it is also possible to react polymers with other arylene units having up to 20 carbon atoms, such as naphthylene or anthraquinone units, or having substituted phenylene units whose substituents have from 1 to 10 carbon atoms, or having heteroaromatic arylene units.
The concentration of the nitric acid according to the invention preferably used for oxidizing the polyarylene sulfides is from 60 to 80%, preferably from 65 to 75%, particularly preferably from 68 to 69.99%.
The temperature at which the novel oxidation of the polyarylene sulfides is carried out with nitric acid is from 60 to 120xc2x0 C., preferably from 75 to 100xc2x0 C., particularly preferably from 80 to 90xc2x0 C.
The period for which the novel oxidation of polyarylene sulfide to polyarylene sulfoxide is carried out is from 30 to 120 min, preferably from 60 to 80 min.
The amount of nitric acid needed for complete oxidation of the polyarylene sulfide is adjusted to give a molar ratio of HNO3 to polyarylene sulfide of from 5:1 to 20:1, preferably from 9.0:1 to 11:1, particularly preferably from 9.5:1 to 10.5:1.
According to the invention, air or oxygen may also be passed through the reactor so that NO produced during the oxidation reaction is further oxidized directly to NO2, which advantageously and at low industrial cost can be condensed and reclaimed.
According to the invention the polyarylene sulfoxide produced is isolated from the reaction mixture by cooling the reaction mixture to a temperature of from 60 to 70xc2x0 C. after the reaction time has expired, whereupon polyarylene sulfoxide produced by the oxidation begins to precipitate from a temperature as high as 75xc2x0 C., and then diluting the cooled reaction mixture with water while stirring continues. Once the mixture has reached a temperature of from 20 to 50xc2x0 C. after further cooling the product may be filtered off, washed with water until neutral and, for example, dried in a drying cabinet or dryer.
Since the first filtrate has a very high concentration of acid it may, if desired, be worked up again to give nitric acid of the desired concentration for the oxidation reaction and thus reintroduced into the reaction loop.